The success of waterless planography is dependent upon there being a low free surface energy polymer overlaying the non-print areas of the printing plate. The polymer prevents ink from wetting the non-printing areas, thus clearly delineating the image areas which accept the ink.
The process will only work when the ink has sufficient viscosity defined by the properties of "tack" (viscoelasticity), and "body" (non-Newtonian plastic viscosity). Inks having these properties, are repelled by the low free surface energy polymer that overlays the non-print areas of the plate.
In actual practice, however, inks having the sufficient "tack" and "body" properties, are often too viscous, and will rupture paper media used on most printing presses.
Another problem with the printing inks results from the high speed of the printing operation. The high speed rollers generate a considerable amount of heat, which will quickly reduce ink viscosities to the point where they will not work. Therefore, many systems have installed water-cooled rollers to maintain proper ink viscosity. Water-cooled rollers, however, are expensive to install and difficult to operate.
The present invention has developed an article and method for resolving the aforementioned problems.
The invention observes that the "form roller" is responsible for causing the plate to be swept clean of ink tone in the non-printing areas. Normal rollers without ink generally separate from the plate surface with very little pull being exerted between the roller and plate. Hence, it is realized that only the ink provides the pulling force at separation.
The invention has discovered that should the roller be fabricated in such a manner as to be sufficiently tacky, then this will allow the printing to be accomplished free of tone, while using low viscosity inks. In other words, the rollers now replace the function of "tackiness" and/or tone removal originally the province of high viscosity inks. This observation and discovery flies in the face of conventional practice and wisdom, since tacky rollers are believed to hinder the printing process.
The use of low viscosity inks with the "tacky" roller of this invention, will in turn allow for the printing operation to be accomplished without the need for water-cooled rollers. This is so, because even though the inks may become less viscous as the rollers generate heat and raise the temperature of the inks, the rollers will remain tacky despite the increased temperature.
The use of the low viscosity inks will also allow the printing operation to use the commercially available paper media without concern that they will rupture during printing.
The "tackiness" provided by the roller article of this invention, has a slightly different property than that observed for inks. Ink "tackiness" can be defined as the resistance to the force of separation of liquids from solid surfaces, a quality similar to liquid surface tension. The "tackiness" of the roller apparatus of this invention, however, is defined as: "pressure-sensitive tack". "Pressure-sensitive tack" can be further defined as the quality or property of the separation of solid surfaces that are in contact with each other, and which separate cleanly.
The invention, having utilized a new property in the fabrication and use of ink rollers, has developed a test by which the "pressure-sensitive tack" of the roller can be quantitatively measured. This is necessary, in order to provide precision and accuracy to the fabrication process, and in the assessment of roller workability.
To measure tack as defined above, the invention has constructed a device as follows: A smooth coated paper (from a single mill run) is cut to a length two inches wide and six inches long. The paper is placed on a clean glass surface with weights placed on the distal ends of the paper at distances one-half inch from the edge at the middle of the paper width. The fabricated roller, whose tackiness is to be measured is placed in contact with the paper at its mid-point. Next, the roller is lifted from the paper. It will be observed that if the roller has sufficient "pressure-sensitive tack", then it will lift the paper, causing it to buckle. The total amount of weight that will hold the paper from being lifted, therefore, is the force that quantitatively defines the quality of "Pressure-sensitive tack".
The invention has found that weights in the range of between 200 and 1,000 grams indicate a "pressure-sensitive tack" sufficient to provide a roller that will remove the ink from the non-image areas of the plate surface.